Mutations in the EPHA2 gene are a major contributor to inherited cataracts in South-Eastern Australia

Mapping the Universe of Eph Receptor and Ephrin Ligand Transcripts in Epithelial and Fiber Cells of the Eye Lens

The eye lens is a transparent, ellipsoid organ in the anterior chamber of the eye that is required for fine focusing of light onto the retina to transmit a clear image. Cataracts, defined as any opacity in the lens, remains the leading cause of blindness in the world. Recent studies in humans and mice indicate that Eph–ephrin bidirectional signaling is important for maintaining lens transparency. Specifically, mutations and polymorphisms in the EphA2 receptor and the ephrin-A5 ligand have been linked to congenital and age-related cataracts. It is unclear what other variants of Ephs and ephrins are expressed in the lens or whether there is preferential expression in epithelial vs. fiber cells. We performed a detailed analysis of Eph receptor and ephrin ligand mRNA transcripts in whole mouse lenses, epithelial cell fractions, and fiber cell fractions using a new RNA isolation method. We compared control samples with EphA2 knockout (KO) and ephrin-A5 KO samples. Our results revealed the presence of transcripts for 12 out of 14 Eph receptors and 8 out of 8 ephrin ligands in various fractions of lens cells. Using specific primer sets, RT-PCR, and sequencing, we verified the variant of each gene that is expressed, and we found two epithelial-cell-specific genes. Surprisingly, we also identified one Eph receptor variant that is expressed in KO lens fibers but is absent from control lens fibers. We also identified one low expression ephrin variant that is only expressed in ephrin-A5 control samples. These results indicate that the lens expresses almost all Ephs and ephrins, and there may be many receptor–ligand pairs that play a role in lens homeostasis.

Pathogenic genetic variants identified in Australian families with paediatric cataract

Objective

Paediatric (childhood or congenital) cataract is an opacification of the normally clear lens of the eye and has a genetic basis in at least 18% of cases in Australia. This study aimed to replicate clinical gene screening to identify variants likely to be causative of disease in an Australian patient cohort.

Methods and analysis

Sixty-three reported isolated cataract genes were screened for rare coding variants in 37 Australian families using genome sequencing.

Results

Disease-causing variants were confirmed in eight families with variant classification as ‘likely pathogenic’. This included novel variants PITX3 p.(Ter303LeuextTer100), BFSP1 p.(Glu375GlyfsTer2), and GJA8 p.(Pro189Ser), as well as, previously described variants identified in genes GJA3, GJA8, CRYAA, BFSP1, PITX3, COL4A1 and HSF4. Additionally, eight variants of uncertain significance with evidence towards pathogenicity were identified in genes: GJA3, GJA8, LEMD2, PRX, CRYBB1, BFSP2, and MIP.

Conclusion

These findings expand the genotype–phenotype correlations of both pathogenic and benign variation in cataract-associated genes. They further emphasise the need to develop additional evidence such as functional assays and variant classification criteria specific to paediatric cataract genes to improve interpretation of variants and molecular diagnosis in patients.

Roles of Eph-Ephrin Signaling in the Eye Lens Cataractogenesis, Biomechanics, and Homeostasis

The eye lens is responsible for fine focusing of light onto the retina, and its function relies on tissue transparency and biomechanical properties. Recent studies have demonstrated the importance of Eph-ephrin signaling for the maintenance of life-long lens homeostasis. The binding of Eph receptor tyrosine kinases to ephrin ligands leads to a bidirectional signaling pathway that controls many cellular processes. In particular, dysfunction of the receptor EphA2 or the ligand ephrin-A5 lead to a variety of congenital and age-related cataracts, defined as any opacity in the lens, in human patients. In addition, a wealth of animal studies reveal the unique and overlapping functions of EphA2 and ephrin-A5 in lens cell shape, cell organization and patterning, and overall tissue optical and biomechanical properties. Significant differences in lens phenotypes of mouse models with disrupted EphA2 or ephrin-A5 signaling indicate that genetic modifiers likely affect cataract phenotypes and progression, suggesting a possible reason for the variability of human cataracts due to Eph-ephrin dysfunction. This review summarizes the roles of EphA2 and ephrin-A5 in the lens and suggests future avenues of study.

EphA2 Affects Development of the Eye Lens Nucleus and the Gradient of Refractive Index

Purpose

Our studies in mouse eye lenses demonstrate that ephrin-A5 and EphA2 are needed for normal epithelial cells and lens transparency. We sought to determine whether EphA2 and ephrin-A5 are important for lens morphometrics, nucleus formation, and refractive index.

Methods

We performed tissue morphometric measurements, electron microscopy, Western blots, and interferometric measurements using an X-ray synchrotron beam source to measure the gradient of refractive index (GRIN) to compare mouse lenses with genetic disruption of EphA2 or ephrin-A5.

Results

Morphometric analysis revealed that although there is no change in the overall lens volume, there is a change in lens shape in both EphA2-/- lenses and ephrin-A5-/- lenses. Surprisingly, EphA2-/- lenses had small and soft lens nuclei different from hard lens nuclei of control lenses. SEM images revealed changes in cell morphology of EphA2-/- fiber cells close to the center of the lens. Inner EphA2-/- lens fibers had more pronounced tongue-and-groove interdigitations and formed globular membrane morphology only in the deepest layers of the lens nucleus. We did not observe nuclear defects in ephrin-A5-/- lenses. There was an overall decrease in magnitude of refractive index across EphA2-/- lenses, which is most pronounced in the nucleus.

Conclusions

This work reveals that Eph-ephrin signaling plays a role in fiber cell maturation, nuclear compaction, and lens shape. Loss of EphA2 disrupts the nuclear compaction resulting in a small lens nucleus. Our data suggest that Eph-ephrin signaling may be required for fiber cell membrane reorganization and compaction and for establishing a normal GRIN.

Genotype, Age, Genetic Background, and Sex Influence Epha2-Related Cataract Development in Mice

Purpose

Age-related cataract is the leading cause of blindness worldwide. Variants in the EPHA2 gene increase the disease risk, and its knockout in mice causes cataract. We investigated whether age, sex, and genetic background, risk factors for age-related cataract, and Epha2 genotype influence Epha2-related cataract development in mice.

Methods

Cataract development was monitored in Epha2+/+, Epha2+/-, and Epha2-/- mice (Epha2Gt(KST085)Byg) on C57BL/6J and FVB:C57BL/6J (50:50) backgrounds. Cellular architecture of lenses, endoplasmic reticulum (ER) stress, and redox state were determined using histological, molecular, and analytical techniques.

Results

Epha2-/- and Epha2+/- mice on C57BL/6J background developed severe cortical cataracts by 18 and 38 weeks of age, respectively, compared to development of similar cataract significantly later in Epha2-/- mice and no cataract in Epha2+/- mice in this strain on FVB background, which was previously reported. On FVB:C57BL/6J background, Epha2-/- mice developed severe cortical cataract by 38 weeks and Epha2+/- mice exhibited mild cortical cataract up to 64 weeks of age. Progression of cataract in Epha2-/- and Epha2+/- female mice on C57BL/6J and mixed background, respectively, was slower than in matched male mice. N-cadherin and β-catenin immunolabeling showed disorganized lens fiber cells and disruption of lens architecture in Epha2-/- and Epha2+/- lenses, coinciding with development of severe cataracts. EPHA2 immunolabeling showed intracellular accumulation of the mutant EPHA2-β-galactosidase fusion protein that induced a cytoprotective ER stress response and in Epha2+/- lenses was also accompanied by glutathione redox imbalance.

Conclusions

Both, Epha2-/- and Epha2+/- mice develop age-related cortical cataract; age as a function of Epha2 genotype, sex, and genetic background influence Epha2-related cataractogenesis in mice.

Functional analysis of deleterious EPHA2 SNPs in lens epithelial cells

Purpose

Ephrin (Eph) receptor A2 (EPHA2) polymorphism has been associated with age-related cataract (ARC) in different populations worldwide, but the mechanisms by which this polymorphism results in the development of ARC are unclear. Here, we chose four EPHA2 single nucleotide polymorphisms (SNPs; rs35903225, rs145592908, rs137853199, and rs116506614) and studied their function in human lens epithelial cells (LECs).

Methods

The four EPHA2 mutants were overexpressed using lentiviral transduction in human LECs. Cells expressing wild-type (WT) and mutated EPHA2 were subjected to quantitative PCR (qPCR), western blot, immunoprecipitation (IP), and transwell migration assay. MG132 and chloroquine were used to inhibit the degradation of the WT and mutated EPHA2. The structural changes induced by rs137853199 were predicted and optimized using Schrödinger software. IP-mass spectrometry (IP-MS) was performed to examine the proteins that directly interact with WT and rs137853199 EPHA2. Sanger sequencing was performed to determine the frequency of rs137853199 in 184 patients with ARC (73 cortical cataracts, 56 nuclear cataracts, and 55 posterior subcapsular cataracts) and 49 normal controls.

Results

Compared with the WT and the other three mutations, the rs137853199 mutation specifically resulted in a significant decrease in the expression of EPHA2. We identified that EPHA2 rs137853199 is degraded via the ubiquitin-proteasomal pathway through a lysine-48 (K48) residue linkage. Furthermore, the knockdown of EPHA2 reduced cell migration; while the overexpression of WT EPHA2 rescued this defect, the overexpression of rs137853199 EPHA2 did not. In addition, in cells overexpressing rs137853199 EPHA2, the expression of β-catenin, a key protein that regulates cell migration, significantly decreased. We predicted that rs137853199 would induce a conformational change at a linker position in the carboxyl terminal of EPHA2. The IP-MS results showed that the main molecular functions of the proteins that specifically bind WT or rs137853199 EPHA2 are binding and catalysis, while the main protein class is the protein-modifying enzyme. Finally, we discovered that the minor allele frequency of rs137853199 was significantly higher in cortical cataract patients than it was in normal controls.

Conclusions

In summary, these findings suggest a mechanism by which a point mutation in EPHA2 disrupts protein stability, expedites protein degradation, and decreases cell mobility. Importantly, this mutant is associated with cortical cataracts.

Functional analysis of deleterious EPHA2 SNPs in lens epithelial cells

Purpose

Ephrin (Eph) receptor A2 (EPHA2) polymorphism has been associated with age-related cataract (ARC) in different populations worldwide, but the mechanisms by which this polymorphism results in the development of ARC are unclear. Here, we chose four EPHA2 single nucleotide polymorphisms (SNPs; rs35903225, rs145592908, rs137853199, and rs116506614) and studied their function in human lens epithelial cells (LECs).

Methods

The four EPHA2 mutants were overexpressed using lentiviral transduction in human LECs. Cells expressing wild-type (WT) and mutated EPHA2 were subjected to quantitative PCR (qPCR), western blot, immunoprecipitation (IP), and transwell migration assay. MG132 and chloroquine were used to inhibit the degradation of the WT and mutated EPHA2. The structural changes induced by rs137853199 were predicted and optimized using Schrödinger software. IP-mass spectrometry (IP-MS) was performed to examine the proteins that directly interact with WT and rs137853199 EPHA2. Sanger sequencing was performed to determine the frequency of rs137853199 in 184 patients with ARC (73 cortical cataracts, 56 nuclear cataracts, and 55 posterior subcapsular cataracts) and 49 normal controls.

Results

Compared with the WT and the other three mutations, the rs137853199 mutation specifically resulted in a significant decrease in the expression of EPHA2. We identified that EPHA2 rs137853199 is degraded via the ubiquitin-proteasomal pathway through a lysine-48 (K48) residue linkage. Furthermore, the knockdown of EPHA2 reduced cell migration; while the overexpression of WT EPHA2 rescued this defect, the overexpression of rs137853199 EPHA2 did not. In addition, in cells overexpressing rs137853199 EPHA2, the expression of β-catenin, a key protein that regulates cell migration, significantly decreased. We predicted that rs137853199 would induce a conformational change at a linker position in the carboxyl terminal of EPHA2. The IP-MS results showed that the main molecular functions of the proteins that specifically bind WT or rs137853199 EPHA2 are binding and catalysis, while the main protein class is the protein-modifying enzyme. Finally, we discovered that the minor allele frequency of rs137853199 was significantly higher in cortical cataract patients than it was in normal controls.

Conclusions

In summary, these findings suggest a mechanism by which a point mutation in EPHA2 disrupts protein stability, expedites protein degradation, and decreases cell mobility. Importantly, this mutant is associated with cortical cataracts.

Molecular and Genetic Mechanism of Non-Syndromic Congenital Cataracts. Mutation Screening in Spanish Families

Our purpose was to identify mutations responsible for non-syndromic congenital cataracts through the implementation of next-generation sequencing (NGS) in our center. A sample of peripheral blood was obtained from probands and willing family members and genomic DNA was extracted from leukocytes. DNA was analyzed implementing a panel (OFTv2.1) including 39 known congenital cataracts disease genes. 62 probands from 51 families were recruited. Pathogenic or likely pathogenic variants were identified in 32 patients and 25 families; in 16 families (64%) these were de novo mutations. The mutation detection rate was 49%. Almost all reported mutations were autosomal dominant. Mutations in crystallin genes were found in 30% of the probands. Mutations in membrane proteins were detected in seven families (two in GJA3 and five in GJA8). Mutations in LIM2 and MIP were each found in three families. Other mutations detected affected EPHA2, PAX6, HSF4 and PITX3. Variants classified as of unknown significance were found in 5 families (9.8%), affecting CRYBB3, LIM2, EPHA2, ABCB6 and TDRD7. Mutations lead to different cataract phenotypes within the same family.

EPHA2 Segregates with Microphthalmia and Congenital Cataracts in Two Unrelated Families

EPHA2 is a transmembrane tyrosine kinase receptor that, when disrupted, causes congenital and age-related cataracts. Cat-Map reports 22 pathogenic EPHA2 variants associated with congenital cataracts, variable microcornea, and lenticonus, but no previous association with microphthalmia (small, underdeveloped eye, ≥2 standard deviations below normal axial length). Microphthalmia arises from ocular maldevelopment with >90 monogenic causes, and can include a complex ocular phenotype. In this paper, we report two pathogenic EPHA2 variants in unrelated families presenting with bilateral microphthalmia and congenital cataracts. Whole genome sequencing through the 100,000 Genomes Project and cataract-related targeted gene panel testing identified autosomal dominant heterozygous mutations segregating with the disease: (i) missense c.1751C>T, p.(Pro584Leu) and (ii) splice site c.2826-9G>A. To functionally validate pathogenicity, morpholino knockdown of epha2a/epha2b in zebrafish resulted in significantly reduced eye size ± cataract formation. Misexpression of N-cadherin and retained fibre cell nuclei were observed in the developing lens of the epha2b knockdown morphant fish by 3 days post-fertilisation, which indicated a putative mechanism for microphthalmia pathogenesis through disruption of cadherin-mediated adherens junctions, preventing lens maturation and the critical signals stimulating eye growth. This study demonstrates a novel association of EPHA2 with microphthalmia, suggesting further analysis of pathogenic variants in unsolved microphthalmia cohorts may increase molecular diagnostic rates.

Congenital cataract: a guide to genetic and clinical management

Worldwide 20,000-40,000 children with congenital or childhood cataract are born every year with varying degrees and patterns of lens opacification with a broad aetiology. In most cases of bilateral cataract, a causative genetic mutation can be identified, with autosomal dominant inheritance being most common in 44% of cases. Variants in genes involve lens-specific proteins or those that regulate eye development, thus giving rise to other associated ocular abnormalities. Approximately 15% of cases have systemic features, hence paediatric input is essential to minimise comorbidities and support overall development of children at high risk of visual impairment. In some metabolic conditions, congenital cataract may be the presenting sign, and therefore prompt diagnosis is important where there is an available treatment. Multidisciplinary management of children is essential, including ophthalmic surgeons, orthoptists, paediatricians, geneticists and genetic counsellors, and should extend beyond the medical team to include school and local paediatric visual support services. Early surgery and close follow up in ophthalmology is important to optimise visual potential and prevent amblyopia. Routine genetic testing is essential for the complete clinical management of patients, with next-generation sequencing of 115 genes shown to expedite molecular diagnosis, streamline care pathways and inform genetic counselling and reproductive options for the future.

Lay abstract

Childhood cataract: how to manage patients Cataract is a clouding of the lens in the eye. Cataract occurring in children has many different causes, which may include infections passed from mother to child during pregnancy, trauma, medications and exposure to radiation. In most cases of cataract occurring in both eyes, a genetic cause can be found which may be inherited from parents or occur sporadically in the developing baby itself while in the womb. Cataracts may occur on their own, with other eye conditions or be present with other disorders in the body as part of a syndrome. Genetic testing is important for all children with cataract as it can provide valuable information about cause, inheritance and risk to further children and signpost any other features of the disease in the rest of the body, permitting the assembly of the correct multidisciplinary care team. Genetic testing currently involves screening for mutations in 115 genes already known to cause cataract and has been shown to expedite diagnosis and help better manage children. Genetic counselling services can support families in understanding their diagnosis and inform future family planning. In order to optimise vision, early surgery for cataract in children is important. This is because the brain is still developing and an unobstructed pathway for light to reach the back of the eye is required for normal visual development. Any obstruction (such as cataract) if left untreated may lead to permanent sight impairment or blindness, even if it is removed later. A multidisciplinary team involved in the care of a child with cataract should include ophthalmic surgeons, orthoptists, paediatricians, geneticists and genetic counsellors, and should extend beyond the medical team to include school and local child visual support services. They will help to diagnose and manage systemic conditions, optimise vision potential and help patients and their families access best supportive care.

Novel mutations identified in Chinese families with autosomal dominant congenital cataracts by targeted next-generation sequencing

Background

Congenital cataract is a clinically and genetically heterogeneous visual impairment. The aim of this study was to identify causative mutations in five unrelated Chinese families diagnosed with congenital cataracts.

Methods

Detailed family history and clinical data were collected, and ophthalmological examinations were performed using slit-lamp photography. Genomic DNA was extracted from peripheral blood of all available members. Thirty-eight genes associated with cataract were captured and sequenced in 5 typical nonsyndromic congenital cataract probands by targeted next-generation sequencing (NGS), and the results were confirmed by Sanger sequencing. Bioinformatics analysis was performed to predict the functional effect of mutant genes.

Results

Results from the DNA sequencing revealed five potential causative mutations: c.154 T > C(p.F52 L) in GJA8 of Family 1, c.1152_1153insG(p.S385Efs*83) in GJA3 of Family 2, c.1804 G > C(p.G602R) in BFSP1 of Family 3, c.1532C > T(p.T511 M) in EPHA2 of Family 4 and c.356G > A(p.R119H) in HSF4 of Family 5. These mutations co-segregated with all affected individuals in the families and were not found in unaffected family members nor in 50 controls. Bioinformatics analysis from several prediction tools supported the possible pathogenicity of these mutations.

Conclusions

In this study, we identified five novel mutations (c.154 T > C in GJA8, c.1152_1153insG in GJA3, c.1804G > C in BFSP1, c.1532C > T in EPHA2, c.356G > A in HSF4) in five Chinese families with hereditary cataracts, respectively. NGS can be used as an effective tool for molecular diagnosis of genetically heterogeneous disorders such as congenital cataract, and the results can provide more effective clinical diagnosis and genetic counseling for the five families.

A Human Three-Dimensional In Vitro Model of Lens Epithelial Cells as a Model to Study Mechanisms of Drug-Induced Posterior Subcapsular Cataracts

Purpose: Cataract is a pathological opacification of the lens, which is still one of the leading causes of blindness in the world. Several etiologies are described, among them drug-induced cataract, for example, posterior subcapsular cataract (PSC) after steroid treatment. To investigate different mechanisms of drug-induced cataract a human three-dimensional (3D) lens in vitro model was developed, consisting of immortalized human lens epithelial cells. Methods: These cells were cultivated on 96-well, ultralow attachment plates, where they rapidly form spheroids. By gene expression analysis different markers were observed, which are important to maintain lens transparency, such as ephrin type-A receptor 2 (EphA2) or α-smooth muscle actin (α-SMA). Results: The lens epithelial cells form a spheroid within a few days and show stable expression of important lens marker, and size and viability remain stable up to 26 days in culture. The gene expression of the glucocorticoid-treated spheroids revealed a clear shift in the expression of EphA2, α-SMA, αB-crystallin (CRYAB), and heat shock protein beta-1 (HSPB1). Furthermore, the glucocorticoid treatment did not improve cell survival. Conclusions: This study proposes a useful 3D in vitro model, which expresses important lens markers and is capable of demonstrating features found in drug-induced cataracts. As the viability remains stable over long time, this model can also be used for long-term treatment. The main characteristics are the increased expression of α-SMA, CRYAB, and HSPB1 and the decreased expression of EphA2. The present data provide some first evidence on novel mechanisms involved in glucocorticoid-induced cataracts.

Novel mutations in CRYBB1/CRYBB2 identified by targeted exome sequencing in Chinese families with congenital cataract

AIM

To summarize the phenotypes and identify the underlying genetic cause of the CRYBB1 and CRYBB2 gene responsible for congenital cataract in two Chinese families.

METHODS

Detailed family histories and clinical data were collected from patients during an ophthalmologic examination. Of 523 inheritable genetic vision system-related genes were captured and sequenced by targeted next-generation sequencing, and the results were confirmed by Sanger sequencing. The possible functional impacts of an amino acid substitution were performed with PolyPhen-2 and SIFT predictions.

RESULTS

The patients in the two families were affected with congenital cataract. Sixty-five (FAMILY-1) and sixty-two (FAMILY-2) single-nucleotide polymorphisms and indels were selected by recommended filtering criteria. Segregation was then analyzed by applying Sanger sequencing with the family members. A heterozygous CRYBB1 mutation in exon 4 (c.347T>C, p.L116P) was identified in sixteen patients in FAMILY-1. A heterozygous CRYBB2 mutation in exon 5 (c.355G>A, p.G119R) was identified in three patients in FAMILY-2. Each mutation co-segregated with the affected individuals and did not exist in unaffected family members and 200 unrelated normal controls. The mutation was predicted to be highly conservative and to be deleterious by both PolyPhen-2 and SIFT.

CONCLUSION

The CRYBB1 mutation (c.347T>C) and CRYBB2 mutation (c.355G>A) are novel in patients with congenital cataract. We summarize the variable phenotypes among the patients, which expanded the phenotypic spectrum of congenital cataract in a different ethnic background.

Germ-line and somatic EPHA2 coding variants in lens aging and cataract

Rare germ-line mutations in the coding regions of the human EPHA2 gene (EPHA2) have been associated with inherited forms of pediatric cataract, whereas, frequent, non-coding, single nucleotide variants (SNVs) have been associated with age-related cataract. Here we sought to determine if germ-line EPHA2 coding SNVs were associated with age-related cataract in a case-control DNA panel (> 50 years) and if somatic EPHA2 coding SNVs were associated with lens aging and/or cataract in a post-mortem lens DNA panel (> 48 years). Micro-fluidic PCR amplification followed by targeted amplicon (exon) next-generation (deep) sequencing of EPHA2 (17-exons) afforded high read-depth coverage (1000x) for > 82% of reads in the cataract case-control panel (161 cases, 64 controls) and > 70% of reads in the post-mortem lens panel (35 clear lens pairs, 22 cataract lens pairs). Novel and reference (known) missense SNVs in EPHA2 that were predicted in silico to be functionally damaging were found in both cases and controls from the age-related cataract panel at variant allele frequencies (VAFs) consistent with germ-line transmission (VAF > 20%). Similarly, both novel and reference missense SNVs in EPHA2 were found in the post-mortem lens panel at VAFs consistent with a somatic origin (VAF > 3%). The majority of SNVs found in the cataract case-control panel and post-mortem lens panel were transitions and many occurred at di-pyrimidine sites that are susceptible to ultraviolet (UV) radiation induced mutation. These data suggest that novel germ-line (blood) and somatic (lens) coding SNVs in EPHA2 that are predicted to be functionally deleterious occur in adults over 50 years of age. However, both types of EPHA2 coding variants were present at comparable levels in individuals with or without age-related cataract making simple genotype-phenotype correlations inconclusive.

A recurrent splice-site mutation in EPHA2 causing congenital posterior nuclear cataract

Intoduction: Inherited cataract, opacification of the lens, is the most common worldwide cause of blindness in children. We aimed to identify the genetic cause of autosomal dominant (AD) posterior nuclear cataract in a four generation British family.

METHODS

Whole genome sequence (WGS) was performed on two affected and one unaffected individual of the family and further validated by direct sequencing. Haplotype analysis was performed via genotying.

RESULTS

A splice-site mutation c.2826-9G>A in the gene EPHA2, encoding EPH receptor A2 was identified and found to co-segregate with disease.

CONCLUSIONS

We have identified a recurrent splice-site mutation c.2826-9G>A in EPHA2 causing isolated posterior nuclear cataract, providing evidence of further phenotypic heterogeneity associated with this variant.

High-Throughput Genetic Screening of 51 Pediatric Cataract Genes Identifies Causative Mutations in Inherited Pediatric Cataract in South Eastern Australia

Pediatric cataract is a leading cause of childhood blindness. This study aimed to determine the genetic cause of pediatric cataract in Australian families by screening known disease-associated genes using massively parallel sequencing technology. We sequenced 51 previously reported pediatric cataract genes in 33 affected individuals with a family history (cases with previously known or published mutations were excluded) using the Ion Torrent Personal Genome Machine. Variants were prioritized for validation if they were predicted to alter the protein sequence and were absent or rare with minor allele frequency <1% in public databases. Confirmed mutations were assessed for segregation with the phenotype in all available family members. All identified novel or previously reported cataract-causing mutations were screened in 326 unrelated Australian controls. We detected 11 novel mutations in GJA3, GJA8, CRYAA, CRYBB2, CRYGS, CRYGA, GCNT2, CRYGA, and MIP; and three previously reported cataract-causing mutations in GJA8, CRYAA, and CRYBB2 The most commonly mutated genes were those coding for gap junctions and crystallin proteins. Including previous reports of pediatric cataract-associated mutations in our Australian cohort, known genes account for >60% of familial pediatric cataract in Australia, indicating that still more causative genes remain to be identified.

A novel splice donor site mutation in EPHA2 caused congenital cataract in a Chinese family

Background:

Congenital cataract is a rare disorder characterized by crystallin denaturation, which becomes a major cause of childhood blindness. Although more than fifty pathogenic genes for congenital cataract have been reported, the genetic causes of many cataract patients remain unknown. In this study, the aim is to identify the genetic cause of a five-generation Chinese autosomal dominant congenital cataract family.

Methods:

Whole exome sequencing (WES) was performed on three affected and one unaffected member of the family, known causative genes were scanned first. Sanger sequencing was used to validate co-segregation of the candidate variant in the family. The impact on the transcript and amino acid sequences of the variant was further analyzed.

Results:

We identified a novel splice donor site mutation c. 2825+1G >A in EPHA2 that was absent in public and in-house databases and showed co-segregation in the family. This variant resulted in an altered splice that led to protein truncation.

Conclusions:

The mutation we identified was responsible for congenital cataract in our studied family. Our findings broaden the spectrum of causative mutations in EPHA2 gene for congenital cataract and suggest that WES is an efficient strategy to scan variants in known causative genes for genetically heterogeneous diseases.

EphA2 and ephrin-A5 are not a receptor-ligand pair in the ocular lens

Eph-ephrin bidirectional signaling is essential for eye lens transparency in humans and mice. Our previous studies in mouse lenses demonstrate that ephrin-A5 is mainly expressed in the anterior epithelium, where it is required for maintaining the anterior epithelial monolayer. In contrast, EphA2 is localized in equatorial epithelial and fiber cells where it is essential for equatorial epithelial and fiber cell organization and hexagonal cell shape. Immunostaining of lens epithelial and fiber cells reveals that EphA2 and ephrin-A5 are also co-expressed in anterior fiber cell tips, equatorial epithelial cells and newly formed lens fibers, although they are not precisely colocalized. Due to this complex expression pattern and the promiscuous interactions between Eph receptors and ephrin ligands, as well as their complex bidirectional signaling pathways, cataracts in ephrin-A5(-/-) or EphA2(-/-) lenses may arise from loss of function or abnormal signaling mechanisms. To test whether abnormal signaling mechanisms may play a role in cataractogenesis in ephrin-A5(-/-) or EphA2(-/-) lenses, we generated EphA2 and ephrin-A5 double knockout (DKO) mice. We compared the phenotypes of EphA2(-/-) and ephrin-A5(-/-) lenses to that of DKO lenses. DKO lenses displayed an additive lens phenotype that was not significantly different from the two single KO lens phenotypes. Similar to ephrin-A5(-/-) lenses, DKO lenses had abnormal anterior epithelial cells leading to a large mass of epithelial cells that invade into the underlying fiber cell layer, directly resulting in anterior cataracts in ephrin-A5(-/-) and DKO lenses. Yet, similar to EphA2(-/-) lenses, DKO lenses also had abnormal packing of equatorial epithelial cells with disorganized meridional rows, lack of a lens fulcrum and disrupted fiber cells. The DKO lens phenotype rules out abnormal signaling by EphA2 in ephrin-A5(-/-) lenses or by ephrin-A5 in EphA2(-/-) lenses as possible cataract mechanisms. Thus, these results indicate that EphA2 and ephrin-A5 do not form a lens receptor-ligand pair, and that EphA2 and ephrin-A5 have other binding partners in the lens to help align differentiating equatorial epithelial cells or maintain the anterior epithelium, respectively.

Targeted Exome Sequencing of Congenital Cataracts Related Genes: Broadening the Mutation Spectrum and Genotype-Phenotype Correlations in 27 Chinese Han Families

Congenital cataract is the most frequent inherited ocular disorder and the most leading cause of lifelong visual loss. The screening of pathogenic mutations can be very challenging in some cases, for congenital cataracts are clinically and genetically heterogeneous diseases. The aim of this study is to investigate the mutation spectrum and frequency of 54 cartaract-associated genes in 27 Chinese families with congenital cataracts. Variants in 54 cataract-associated genes were screened by targeted next-generation sequencing (NGS) and then validated by Sanger sequencing. We identified pathogenic variants in 62.96% (17/27) of families, and over 52.94% (9/17) of these variants were novel. Among them, three are splicing site mutations, four are nonsense mutations, seven are missense mutations, two are frame shift mutations and one is intronic mutation. This included identification of: complex ocular phenotypes due to two novel PAX6 mutations; progressive cortical cataract and lamellar cataract with lens subluxation due to two novel CRYGS mutations. Mutations were also found in rarely reported genes including CRYBA4, CRYBA2, BFSP1, VIM, HSF4, and EZR. Our study expands the mutation spectrum and frequency of genes responsible for congenital cataracts. Targeted next-generation sequencing in inherited congenital cataract patients provided significant diagnostic information.

Inherited Congenital Cataract: A Guide to Suspect the Genetic Etiology in the Cataract Genesis

Cataracts are the principal cause of treatable blindness worldwide. Inherited congenital cataract (CC) shows all types of inheritance patterns in a syndromic and nonsyndromic form. There are more than 100 genes associated with cataract with a predominance of autosomal dominant inheritance. A cataract is defined as an opacity of the lens producing a variation of the refractive index of the lens. This variation derives from modifications in the lens structure resulting in light scattering, frequently a consequence of a significant concentration of high-molecular-weight protein aggregates. The aim of this review is to introduce a guide to identify the gene involved in inherited CC. Due to the manifold clinical and genetic heterogeneity, we discarded the cataract phenotype as a cardinal sign; a 4-group classification with the genes implicated in inherited CC is proposed. We consider that this classification will assist in identifying the probable gene involved in inherited CC.

Association between polymorphisms of OGG1, EPHA2 and age-related cataract risk: a meta-analysis

Background

Evidences have identified the correlation of 8-oxoguanine DNA glycosylase-1 (OGG1) and eph-receptor tyrosine kinase-type A2 (EPHA2) polymorphisms in age-related cataract (ARC) risk. However, the results were not consistent. The objective of this study was to examine the role of these two gene polymorphisms in ARC susceptibility.

Methods

Eligible case–control studies published between January 2000 and 2015 were searched and retrieved in the electronic databases. The odds ratio with 95 % confidence interval (CI) was employed to calculate the strength of the relationship.

Results

We totally screened out six articles, including 5971 cataract patients and 4189 matched controls. Three variants were contained (OGG1 rs1052133; EPHA2 rs7543472 and rs11260867). For OGG1 rs1052133, we detected a significant correlation between OGG1 polymorphism and ARC risk under the heterogenous model (CG vs. CC: OR = 1.34, 95 % CI = 1.06–1.70, P = 0.01) and dominant model (GG+CG vs. CC: OR = 1.45, 95 % CI = 1.16–1.81, P = 0.001), especially in patients with cortical cataract of subgroup analysis by phenotypes (P < 0.05). For EPHA2 rs7543472 and rs11260867, we did not find a positive association between these two mutations and ARC susceptibility in total cases. Subgroup analysis by phenotypes of cataract showed that only in cortical cataract, genotypes of rs7543472 under the allele model, homogenous model and recessive model; genotypes of rs11260867 under the heterogenous model and dominant model were associated with ARC risk.

Conclusions

OGG1 rs1052133 (CG and CG+GG genotypes) might be risk factor for ARC, particularly in cortical cataract risk. EPHA2 rs7543472 (T allele and TT genotype) and rs11260867 (CG and GG+CG genotypes) might be associated with cortical cataract.

Distribution of gene mutations in sporadic congenital cataract in a Han Chinese population

PURPOSE

This study aimed to investigate the genetic effects underlying non-familial sporadic congenital cataract (SCC).

METHODS

We collected DNA samples from 74 patients with SCC and 20 patients with traumatic cataract (TC) in an age-matched group and performed genomic sequencing of 61 lens-related genes with target region capture and next-generation sequencing (NGS). The suspected SCC variants were validated with MassARRAY and Sanger sequencing. DNA samples from 103 healthy subjects were used as additional controls in the confirmation examination.

RESULTS

By filtering against common variants in public databases and those associated with TC cases, we identified 23 SCC-specific variants in 17 genes from 19 patients, which were predicted to be functional. These mutations were further confirmed by examination of the 103 healthy controls. Among the mutated genes, CRYBB3 had the highest mutation frequency with mutations detected four times in four patients, followed by EPHA2, NHS, and WDR36, the mutation of which were detected two times in two patients. We observed that the four patients with CRYBB3 mutations had three different cataract phenotypes.

CONCLUSIONS

From this study, we concluded the clinical and genetic heterogeneity of SCC. This is the first study to report broad spectrum genotyping for patients with SCC.

EPHA2 MUTATIONS CONTRIBUTE TO CONGENITAL CATARACT THROUGH DIVERSE MECHANISMS

PURPOSE

Congenital cataract is a leading cause of childhood blindness. Mutations in the EPHA2 gene are one of the causes of inherited congenital cataract. The EPHA2 gene encodes a membrane-bound tyrosine kinase receptor and is highly expressed in epithelial cells, including in the ocular lens. Signaling through the EPHA2 receptor plays a pivotal role in epithelial cell homeostasis. The aim of this study was to determine the effect of congenital cataract causing mutations in the EPHA2 gene on the encoded protein in epithelial cells.

METHODS

The effect of five disease-causing mutations, p.P584L (c.1751C>T), p.T940I (c.2819C>T), p.D942fsXC71 (c.2826-9G>A), p.A959T (c.2875G>A), and p.V972GfsX39 (c.2915_2916delTG), on localization of the protein was examined in two in vitro epithelial cell culture systems: Madin-Darby Canine Kidney (MDCK) and human colorectal adenocarcinoma (Caco-2) epithelial cells. Myc-tagged mutant constructs were generated by polymerase chain reaction (PCR)-based mutagenesis. The Myc-tagged wild-type construct was used as a control. The Myc-tagged wild-type and mutant proteins were ectopically expressed and detected by immunofluorescence labeling.

RESULTS

Two of the mutations, p.T940I and p.D942fsXC71, located within the cytoplasmic sterile-α-motif (SAM) domain of EPHA2, led to mis-localization of the protein to the perinuclear space and co-localization with the cis-golgi apparatus, indicating sub-organellar/cellular retention of the mutant proteins. The mutant proteins carrying the remaining three mutations, similar to the wild-type EPHA2, localized to the cell membrane.

CONCLUSIONS

Mis-localization of two of the mutant proteins in epithelial cells suggests that some disease-causing mutations in EPHA2 likely affect lens epithelial cell homeostasis and contribute to cataract. This study suggests that mutations in EPHA2 contribute to congenital cataract through diverse mechanisms.

Breakdown of interlocking domains may contribute to formation of membranous globules and lens opacity in ephrin-A5(-/-) mice

Ephrin-A5, a ligand of the Eph family of receptor tyrosine kinases, plays a key role in lens fiber cell packing and cell-cell adhesion, with approximately 87% of ephrin-A5(-/-) mice develop nuclear cataracts. Here, we investigated the extensive formation of light-scattering globules associated with breakdown of interlocking protrusions during lens opacification in ephrin-A5(-/-) mice. Lenses from wild-type (WT) and ephrin-A5(-/-) mice between 2 and 21 weeks old were studied with light and electron microscopy, immunofluorescence labeling, freeze-fracture TEM and filipin cytochemistry for membrane cholesterol detection. Lens opacities with various densities were first observed in ephrin-A5(-/-) mice at around 60 days old. Dense cataracts in the mutant lenses were seen primarily in the nuclear region surrounded by transparent cortices from all eyes examined. We confirmed that a majority of nuclear cataracts were dislocated posteriorly and ruptured the thinner posterior lens capsule. SEM analysis indicated that numerous interlocking protrusions and wavy ridge-and-valley membrane surfaces in deep cortical and nuclear fibers did not cause lens opacity in both transparent ephrin-A5(-/-) and WT mice. In contrast, abundant isolated membranous globules of approximately 1000 nm in size were distributed randomly along the intact fiber cells during early stage of all ephrin-A5(-/-) cataracts examined. A further examination using both SEM and TEM revealed that isolated globules were generated from the disintegrated interlocking protrusions originally located along the corners of hexagonal fiber cells. Freeze-fracture TEM further revealed the association of square-array aquaporin junctions with both isolated globules and interlocking membrane domains. This study reports for the first time that disrupted interlocking protrusions are the source of numerous large membranous globules that contribute to light scattering and nuclear cataracts in the ephrin-A5(-/-) mice. Our results further suggest that dissociations of N-cadherin and adherens junctions in the associated interlocking domains may result in the formation of isolated globules and nuclear opacities in the ephrin-A5(-/-) mice.

Cataract

Cataract is the leading cause of reversible blindness and visual impairment globally. Blindness from cataract is more common in populations with low socioeconomic status and in developing countries than in developed countries. The only treatment for cataract is surgery. Phacoemulsification is the gold standard for cataract surgery in the developed world, whereas manual small incision cataract surgery is used frequently in developing countries. In general, the outcomes of surgery are good and complications, such as endophthalmitis, often can be prevented or have good ouctomes if properly managed. Femtosecond laser-assisted cataract surgery, an advanced technology, can automate several steps; initial data show no superiority of this approach over current techniques, but the results of many large clinical trials are pending. The greatest challenge remains the growing 'backlog' of patients with cataract blindness in the developing world because of lack of access to affordable surgery. Efforts aimed at training additional cataract surgeons in these countries do not keep pace with the increasing demand associated with ageing population demographics. In the absence of strategie that can prevent or delay cataract formation, it is important to focus efforts and resources on developing models for efficient delivery of cataract surgical services in underserved regions. For an illustrated summary of this Primer, visit: http://go.nature.com/eQkKll.

Phenotypes of Recessive Pediatric Cataract in a Cohort of Children with Identified Homozygous Gene Mutations (An American Ophthalmological Society Thesis)

PURPOSE

To assess for phenotype-genotype correlations in families with recessive pediatric cataract and identified gene mutations.

METHODS

Retrospective review (2004 through 2013) of 26 Saudi Arabian apparently nonsyndromic pediatric cataract families referred to one of the authors (A.O.K.) and for which recessive gene mutations were identified.

RESULTS

Fifteen different homozygous recessive gene mutations were identified in the 26 consanguineous families; two genes and five families are novel to this study. Ten families had a founder CRYBB1 deletion (all with bilateral central pulverulent cataract), two had the same missense mutation in CRYAB (both with bilateral juvenile cataract with marked variable expressivity), and two had different mutations in FYCO1 (both with bilateral posterior capsular abnormality). The remaining 12 families each had mutations in 12 different genes (CRYAA, CRYBA1, AKR1E2, AGK, BFSP2, CYP27A1, CYP51A1, EPHA2, GCNT2, LONP1, RNLS, WDR87) with unique phenotypes noted for CYP27A1 (bilateral juvenile fleck with anterior and/or posterior capsular cataract and later cerebrotendinous xanthomatosis), EPHA2 (bilateral anterior persistent fetal vasculature), and BFSP2 (bilateral flecklike with cloudy cortex). Potential carrier signs were documented for several families.

CONCLUSIONS

In this recessive pediatric cataract case series most identified genes are noncrystallin. Recessive pediatric cataract phenotypes are generally nonspecific, but some notable phenotypes are distinct and associated with specific gene mutations. Marked variable expressivity can occur from a recessive missense CRYAB mutation. Genetic analysis of apparently isolated pediatric cataract can sometimes uncover mutations in a syndromic gene. Some gene mutations seem to be associated with apparent heterozygous carrier signs.

Brain damage and congenital cataract due to autogenously fecal peritonitis in pregnant Wistar rats

PURPOSE

To investigate the morphological aspects of brain and eyes in newborn rats whose mother underwent autogenously fecal peritonitis.

METHODS

Four pregnant rats that underwent fecal peritonitis, with a 10% fecal suspension in dose of 4 ml per kilogram received two antimicrobial treatments: 1. intraperitoneal moxifloxacin and dexamethazone; and 2. Intravenous meropenem. After head inspection, the brain consistencies and the eyes belonging to all offspring were analyzed.

RESULTS

The brains of newborn from rats that received 4 ml/kg of 10% suspension of feces showed, significantly smaller and less than the firm consistency of those in the control group. Congenital cataract was observed in 9 (34.6%). No cataract was observed in the 20 newborn rats from the mothers that received the combination of moxifloxacin and dexamethasone. Cataract could be observed in three (13.6%) offspring from mothers that received meropenem.

CONCLUSIONS

Peritonitis can produce brain damage and congenital cataract in rats. The translation to humans is that intra abdominal infection in pregnant women may be associated with damage in brain and eye structures of their concepts. This can be averting using the adequate early therapeutically approach.

A novel P20R mutation in the alpha-B crystallin gene causes autosomal dominant congenital posterior polar cataracts in a Chinese family

Background

To identify the genetic defects and investigate the possible mechanism of cataract genesis in a five-generation family with autosomal dominant congenital posterior polar cataracts.

Methods

Clinical data were collected, and the lens phenotypes of the affected members in this family were recorded by slit lamp photography. Genomic DNA was isolated from peripheral blood using QIAamp DNA Blood Mini Kits. Twenty-three mutational hot spots associated with autosomal dominant congenital posterior polar cataracts were screened by PCR-based DNA sequencing. Properties and structural models of wild-type and mutant alpha-B (αB)-crystallin (CRYAB) were generated and analyzed using SWISS-MODEL.

Results

All affected individuals in this family started to exhibit poor vision at the age of 8–10 years. The lens opacity consisted of a single, well-defined plaque, 0.5–3 mm in diameter, which was confined to the posterior pole of the lens. DNA sequencing analysis of the affected members showed a novel, heterozygous missense mutation c.59C > G (P20R) in exon 1 of the CRYAB gene. This mutation was not found in 10 unaffected family members, or in 200 unaffected and unrelated individuals, thereby excluding the possibility that it is a rare polymorphism. Data generated using the ProtScale and PyMOL programs revealed that the mutation altered the stability and solubility of the αB-crystallin protein.

Conclusions

This study reported a novel c.59C > G (P20R) missense mutation in CRYAB in a five-generation Chinese family with posterior polar cataract.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2415-14-108) contains supplementary material, which is available to authorized users.

Exome sequencing of 18 Chinese families with congenital cataracts: a new sight of the NHS gene

PURPOSE

The aim of this study was to investigate the mutation spectrum and frequency of 34 known genes in 18 Chinese families with congenital cataracts.

METHODS

Genomic DNA and clinical data was collected from 18 families with congenital cataracts. Variations in 34 cataract-associated genes were screened by whole exome sequencing and then validated by Sanger sequencing.

RESULTS

Eleven candidate variants in seven of the 34 genes were detected by exome sequencing and then confirmed by Sanger sequencing, including two variants predicted to be benign and the other pathogenic mutations. The nine mutations were present in 9 of the 18 (50%) families with congenital cataracts. Of the four families with mutations in the X-linked NHS gene, no other abnormalities were recorded except for cataract, in which a pseudo-dominant inheritance form was suggested, as female carriers also had different forms of cataracts.

CONCLUSION

This study expands the mutation spectrum and frequency of genes responsible for congenital cataract. Mutation in NHS is a common cause of nonsyndromic congenital cataract with pseudo-autosomal dominant inheritance. Combined with our previous studies, a genetic basis could be identified in 67.6% of families with congenital cataracts in our case series, in which mutations in genes encoding crystallins, genes encoding connexins, and NHS are responsible for 29.4%, 14.7%, and 11.8% of families, respectively. Our results suggest that mutations in NHS are the common cause of congenital cataract, both syndromic and nonsyndromic.

Identification of a novel C-terminal extension mutation in EPHA2 in a family affected with congenital cataract

PURPOSE

Congenital cataracts occur in 3-4 per 10,000 live births and account for 5% to 20% of pediatric blindness worldwide. With more than 37 genes known to be associated with isolated congenital cataract, whole exome sequencing (WES) was recently introduced as an efficient method for screening all known factors.

METHODS

Whole exome analysis in two members of a four-generation pedigree affected with dominant congenital cataract and glaucoma was performed by WES; co-segregation analysis of identified variants in all pedigree members was completed by Sanger sequencing.

RESULTS

Analysis of the WES data identified a novel pathogenic variant in EPHA2, c.2925dupC, p.(Ile976Hisfs*37), that demonstrated complete cosegregation with the phenotype in the pedigree. The mutation occurs in the final amino acid before the stop codon of the normal EPHA2 protein and is predicted to produce a mutant protein with an erroneous C-terminal extension of 35 amino acids. Nine other families have been previously reported with dominant congenital/juvenile cataracts and mutations in EPHA2. Two additional likely loss-of-function variants in genes known to cause dominant congenital cataract were considered and excluded based on control data and cosegregation analysis: a nonsense variant in CYRBB3, c.547G>T, p.(Glu183*), and a splicing variant in CRYBA2, c.446+1G>A.

CONCLUSIONS

Identification of a novel pathogenic EPHA2 allele further implicates this gene in congenital cataract. This is only the second EPHA2 mutation that specifically affects the most C-terminal PSD95/Dlg/ZO1 (PDZ)-binding motif and the third pathogenic allele associated with an erroneous C-terminal extension beyond the normal stop codon.